Simplex data communication systems, also known as one-way or passive systems, are employed in connection with paging. Generally speaking, one or more transmitters broadcast data communications. The communications include data which identify specific pagers. A population of pagers continually receive the broadcast communications. Conventional paging systems experience several problems, one of which is related to a limited range capability. A paging system works only when its pagers reside within the area covered by the system's transmitters. When subscribers travel outside this area, their pagers cannot receive calls. Another problem with conventional paging systems is concerned with multipathing, interference, signal reflections, and the like. Pagers are worn by users who often place themselves inside automobiles, inside buildings, near large grounded structures, and in other spaces that electromagnetic paging signals have trouble penetrating. Consequently, the pagers' ability to receive communications is poor in urban areas, which typically include many interfering structures.
To address the limited range and interference problems, some paging systems employ a simulcast technique wherein multiple simultaneously operated transmitters are positioned at spaced apart locations. An increase in power resulting from multiple transmitters and a diversity of signal propagation paths from spaced apart transmitters improve reception. In addition, the spacing between transmitters extends the coverage area by the spacing distance. While this solution is practical in urban areas, it is far too costly for extending paging services to include less populated areas with the urban areas.
The use of satellites in paging has been proposed. The use of satellites may address the limited range problem of conventional pager systems. However, the use of satellites presents its own problems. For example, satellites are typically constrained to using only low-power transmissions. Accordingly, satellites are currently used only to broadcast to terrestrial repeaters which can then rebroadcast high power signals to nearby pagers. This accomplishes nothing toward extending the range beyond that achieved by the terrestrial repeaters because the current page receiving equipment, or pagers, cannot receive the satellites' signals. In addition, a double allotment of spectrum is required for delivery of pages. One communication link delivers page communications from satellites to terrestrial repeaters and another communication link delivers page communications from the terrestrial repeaters to the pagers.
A proposal has been made to integrate a satellite with terrestrial transmitters in a simulcast paging system. However, such a simulcast paging system requires a satellite to be positioned in a geostationary orbit. Since geostationary orbits are achieved at around 35,000 km (22,000 miles) above the earth, signals broadcast from stationary satellites are weak at the surface of the earth. Reception at the surface of the earth, without the use of large antennas for receiving the signals would be unreliable. Accordingly, pagers in such a system need to incorporate impractically large and expensive receiving equipment or forgo any simulcast benefit from satellite transmissions.
Another proposal has been made to utilize dual-mode pagers which can receive both satellite-transmitted pages and terrestrial-transmitted pages. In this system, the satellite-transmitted pages are rebroadcast as terrestrial-transmitted pages. Satellites may be placed in low earth, moving orbits. Accordingly, terrestrial-transmitted pages use a different communication link than the satellite-transmitted pages to prevent interference with the satellite-transmitted pages. Moreover, except in the interference-prone urban areas where terrestrial-transmitted pages are available, pagers may reliably receive satellite-transmitted signals because these signals originate only a few hundred miles above the earth. On the other hand, the use of dual-mode receivers increases the size, weight, power consumption, and cost of the pager. In addition, the amount of spectrum needed to transmit pages is twice that required to transmit pages over only a single communication link.